Up to the present time, it has been proposed to use either germanium or gallium arsenide as the substrate for solar cells in which the principal active junction is formed of n-type and p-type gallium arsenide. Substrates of gallium arsenide have been preferred because of their electrical properties in view of the problems encountered with germanium substrates. These problems have in part involved the so-called "cascade effect", in which some of the total output arises from the junction of gallium arsenide with the germanium substrate, which is particularly responsive to infrared energy, and which has a relatively high temperature coefficient.
However, germanium would be preferred as a substrate for supporting gallium arsenide solar cells for a number of reasons. First, germanium has a greater fracture toughness than gallium arsenide, as a substrate. For example, an 8 mil (0.2 mm) thick germanium wafer is twice as strong as a 12 mil (0.3 mm) thick gallium arsenide wafer. Cost is another factor favoring germanium, since it is 40 to 50% less expensive than gallium arsenide. Finally, germanium wafers used for solar cells at 4 mils (0.1 mm) thickness are 66% lighter in weight than gallium arsenide at 12 mils (0.3 mm) thickness. Weight is an important factor for space applications, for example.
Yet, in addition to the cascade effect, germanium also evidences the so-called "self-doping" effect, which occurs at high temperatures when the germanium substrate is exposed to the gases used to deposit gallium arsenide. With germanium having a melting point of about 937.degree. C., and with vapor deposition of gallium arsenide taking place at temperatures of up to 780.degree. C., some germanium may be volatilized and adversely affect the critical doping of other solar cells being processed within the same enclosed chamber during production manufacturing. This effect has in the past been blocked by a special cap of gallium arsenide, involving a process which is relatively costly.
U.S. Pat. No. 4,915,744, issued Apr. 10, 1990, to Frank Ho et al and assigned to the same assignee as the present application, discloses and claims a gallium arsenide solar cell formed on a germanium substrate cut at a special angle, with its surface generally perpendicular to the [001] axis, but tilted by about 6.degree. to 15.degree. toward the direction generally about half-way between the [011] and [111] axial directions. To avoid the cascade effect, the junction with the substrate may be passivated or photovoltaically inhibited by initiating vapor deposition of GaAs at a temperature below 700.degree. C. and then rapidly ramping the temperature up to a high vapor deposition temperature, then back down to normal vapor deposition temperatures. Poisoning of the GaAs layer by germanium may be prevented expensively by using a silicon dioxide coating on one side of the germanium substrate.
The foregoing patent is certainly useful for its intended purpose. However, attempts continue to develop solar cells that efficiently utilize as much of the solar spectrum as possible. For example, in order to capture as many photons from the spectrum of solar radiation as possible, the semiconductor material used in the solar cell should be designed with a small bandgap, since the semiconductor is otherwise transparent to radiation with photon energy less than the bandgap. Use of a small bandgap semiconductor material permits photons from lower energy radiation to excite electrons to jump the bandgap. However, at least two negative effects are realized in such a situation. First, the small bandgap results in a low photovoltage device, with low power output. Second, the photons from higher energy radiation will produce many hot carriers with much excess energy that will be lost as heat upon almost immediate thermalization of these hot carriers to the edge of the conduction band.
Yet, on the other hand, if the solar cell is constructed with a larger bandgap to increase the photovoltage and reduce energy loss by thermalization of hot carriers, then the photons from lower energy radiation will not be absorbed, and thus the radiation from the solar spectrum will not be fully utilized.
A solution to the foregoing problem is disclosed and claimed in U.S. Pat. No. 4,667,059, issued May 19, 1987, to Jerry Olson. Essentially, a multijunction (cascade) tandem photovoltaic solar cell device comprises a Ga.sub.x In.sub.1-x P (where 0.505.ltoreq.x.ltoreq.0.515) top cell semiconductor lattice-matched to a GaAs bottom cell semiconductor at a low resistance heterojunction, preferably a p.sup.+ /n.sup.+ heterojunction between the cells. The top and bottom cells are both lattice-matched and current matched for high efficiency solar radiation conversion to electrical energy.
However, the drawback to using a p+/n+ heterojunction between the cells is as follows: It is more difficult to achieve good p+/n+ tunnel diode characteristics (i.e., low tunnel resistance value with high peak tunnel-current density) when the bandgap value of the semiconductor is high. The GaInP layer has bandgap value up to more than 1.9 eV when heavily doped (a necessary requirement for a good tunnel diode), which is much higher than the bandgap of GaAs. Hence, a GaInP/GaAs tunnel diode will not be able to achieve as high a peak tunnel-current density as a GaAs/GaAs tunnel diode. This will reduce the energy conversion efficiency of the cascade solar cell, especially for cells with high current density (i.e., concentrator cells).
Consequently, investigations continue to develop high efficiency solar cells.